There's three main endgame amulets all of whom have their specific scenarios to use. In order of general DPS provided they're Runic Valor Amulet >/=Stargem Stormcaller Necklace > Skain bloodthorn necklace, as part of the set. The Skain triumphs on Mord, Hrung, DLs. The Stargem wins on Summoning Chambers bosses and the annoying 210 Dragon. On every other legacy, ring bosses the runic outshines. On bosses with equal elemental resists like prot/necro both runic and stargem are somewhat equal. Unless you only have average attk to begin with wherein the stargem skill would make a huge difference. Otherwise it won't be a factor in getting the kill or getting the L.

The Osan necklace is amazing for swap cast for shadowstrike. It adds dex n cunning both of which would be low on typical str attk rogue, plus huge skill pts. I've been using it from day 1 of that event when majority of the rogues balked at the idea only to be more into it later on

Bragging aside, you would want both DMG and ATTK on you amulet. The Runic adds 240 heat dmg and 300 attk. Stargem gives 120 cold and ~76 physical dmg(off 200 str) in addition to str skills dmg. See the pattern here? The Scoundrels ammy gives you only str and whatever dmg u gain off it, no attk. Same reason with the trailblazer ammy some people are curious about. It has 450 attk n like 50sh dmg off its 150 str. The runic/stargem has like 4-5 TIMES more damage addition with only a couple hundred less attk. Its not even a comparison